There are three main typical techniques to achieve an extended depth of field (hereinafter referred to as EDOF). The first technique (See Non Patent Literature 1) employs an optical element, referred to as a phase plate, inserted in the optical system in order to make a blur uniform in the scene depth direction. Then, the technique executes image-restoration processing on an image obtained through the blur uniformity, using a previously-measured blur pattern or a calculated blur pattern based on a simulation. Hence, the technique generates an EDOF image. This technique is introduced as the Wave-front coding (hereinafter referred to as the WFC).
The second technique (See Non Patent Literature 2) employs an aperture of which pattern is modified, so that the distance to the focal plane is accurately measured for each of subregions of the image. Then, the technique executes image-restoration processing on each subregion, using a blur pattern which is based on each of previously-measured distances to a corresponding one of the subregions. Hence, the technique generates an EDOF image. This technique is introduced as the Coded Aperture (hereinafter referred to as the CA).
The third technique (See Non Patent Literature 3) involves shifting a focus lens or an imaging device during the exposure time in order to convolve images which are uniformly focused in the scene depth direction (in other words, obtaining a uniform blur in the scene depth direction). Then, the technique executes image-restoration processing on the image obtained through convolution, using a previously-measured blur pattern or a calculated blur pattern based on a simulation. Hence, the technique generates an EDOF image. This technique is introduced as the Flexible DOF (hereinafter referred to as the F-DOF).
There are other techniques than the above techniques. One of the techniques (Non Patent Literature 4) involves estimating the depth and detecting the sharpness of the image, taking advantage of the on-axis chromatic aberration, and generating an all-focus image with image processing. Another technique (Non Patent Literature 5) involves making a uniform blur in the scene depth direction using a multifocal lens, and executing image-restoration processing on the image obtained through the uniformity using a previously-measured blur pattern or a calculated blur pattern based on a simulation. Compared with the first three techniques, however, the next two techniques fail to achieve as large an EDOF as the three techniques do.
In addition, there has been another technique referred to as the focal stack. This technique involves obtaining images each having a different focal point (focal position), and extracting a region-to-be-focused from each of the images. Then, the technique composes the extracted images to generate an EDOF image. Unfortunately, the technique requires many images to be obtained. Thus, the technique inevitably needs a relatively-long time period for obtaining the images, and occupies too much memory.
Various kinds of phase plates are proposed for one of the first three techniques, the WFC. Among the phase plates, the cubic phase mask (hereinafter referred to as the CPM) and the free-form phase mask (hereinafter referred to as the FPM) are introduced as the phase plates for obtaining the largest EDOF. In view of the image quality of the restored image (fewer aritifacts), the FPM is more promising than the CPM (Non Patent Literature 6).
As a weakness of the WFC, however, the phase plate inserted in the optical system tends to deteriorate the off-axis performance of the lens (Non Patent Literature 7). Specifically, the WFC cannot obtain as much a uniform blur with respect to incident light coming from other than the front as a uniform blur with respect to incident light coming from the front. As a result, when an image is restored with a use of an on-axis blur pattern, the off-axis quality of the restored image inevitably deteriorates.
The second technique among the first three techniques; namely the CA, employs an aperture having a modified pattern in order to increase the accuracy of the distance measurement. Due to the modified pattern inherent in the aperture of the technique, specific frequency components are lost from an obtained image and a restored image. In other words, the technique suffers image deterioration. Furthermore, the technique is not suitable for imaging in the dark since an amount of received light in the technique is typically less than that in an ordinary technique no matter how the shape of the aperture is to be modified.
The third of the first three techniques, the F-DOF, enjoys the most excellent image quality among all the three techniques, and achieves a large EDOF. The off-axis performance depends on the performance of the lens itself, which makes it easy to enhance the performance of the imaging apparatus. As an optical condition, however, the technique needs to employ an image-space telecentric lens since the same object needs to be convolved on the same position of the image even though the focal position shifts during the exposure.
The oldest application of the above EDOF technique is the one to microscopes. In the case of a microscope, the focal stack technique has long been used because a user can take time to obtain an image of a still object. The focal stack technique, however, requires much time and work as described above. Hence the EDOF has been disclosed in some references along with the F-DOF technique (Patent Literatures 1 to 4). When the F-DOF is used for the microscope, disclosed are the cases where, during the exposure, (i) a specimen; namely the object, is moved and (ii) the microscope tube is moved.
Based on the premise of image-restoration processing after the exposure, it is reasonable to control the move such that a uniform blur is formed at all times on the object, since an image-restoration processing technique employing a single blur pattern is available (Patent Literature 5). In order to control the move, the object to be moved should be moved at a constant speed in the case where the object is the imaging device. In the case where the focus lens is moved, the focus needs to be shifted as fast as the image plane shifting at a constant speed (Non Patent Literature 3). It is noted that the focus lens may be shifted from the far-end focal point to the near-end focal point and visa versa.
Recently, the EDOF has also been applied to a camera for cellular phones. The use of the EDOF technique for the camera contributes to making the camera smaller. In other words, the EDOF successfully obtains an all-focus image (all the objects in the image are focused) without an autofocus system.
In view of the application of the above techniques, the F-DOF itself is not adopted since the F-DOF requires a mechanism to shift the focus lens or the imaging device. Hence adopted is the WFC or the technique utilizing the on-axis chromatic aberration.
Another application to be considered is the one to regular digital still cameras and digital video cameras. In recent years, users have been looking for more user-friendly and further foolproof digital still cameras and digital video cameras. The EDOF technique is promising since the technique achieves an all-focus image, freeing a user from obtaining an out-of-focus image.
In view of the application, the most excellent technique of all of the above techniques is the F-DOF since the F-DOF has the following features:(i) high image quality is available, (ii) the EDOF effect and the range of focus can be changed at the user's option, (iii) the technique is feasible with the application of a regular autofocus mechanism (no special optical system is required), and (iv) the user can easily switch between EDOF shooting and the regular shooting.